Wave filter impedance-coupled amplifier



Patented Aug. 2, 1932 PATENT orlcs WINFIELD w. sALIsBIIRY, or BnnKELfix, CALIFORNIA WAVE FILTER IMPEDANCE-COUPLED AMPLIFIER Application filed Marchv 19, 1928. Serial No. 262,880.

This invention relates to a vacuum tube amplifier for use in connection with radios, telephone repeaters, television, picture transmission, or any place where a vacuum tube amplifier is required to amplify a definite band of frequencies.

The principal object of the invention is to provide a wave filter in the plate circuit of a vacuum tube.

The invention possesses various objects and features of advantage, some of which, with the foregoing, will be made manifest in the following description of the preferred form of the invention which is illustrated in the drawing accompanying and forming part of the specificationt It is to be understood that it is not intended to limit the invention to the embodiment shown by said drawing and descriptions, as

variations may be adapted within the scope of the invention as set forth in the claims.

In-the accompanying drawing: I

Figure l is a diagrammatic drawing showing a preferred method of coupling a wave filter to the vacuum tube.

Figure 2 is a diagrammatic drawing illustrating the same principle as shown in Figure 1, but used in connection with a four element tube.

Figures 3, a and 5 are drawings illustrating some of the different kinds of wave filters that may be used in the invention.

This type of amplification comes under the general class of impedance-coupled amplifiers. A common example of this is resistance coupling in which the plate current is fed through a high resistance, that is a resistance several times the plate resistance of the tubes, and the energy transfer to the next tube or to the output tubes takes place through a condenser which must be large enough to have an impedance that is low compared to the output impedance for the frequencies to be amplified. This requires a very high voltage source for plate current and is very inselective with regard to the frequencies amplified and thuls is very uneconomical and hard to contro 7 Another method uses a heavy inductance in the plate circuit. This has the advantage of requiring a plate supply having a voltage very little greater than that required for the tube, but its behavior with frequency is poor,

as the inductance presents a different impedance to every different frequency, and hence piveiry difierent frequency is difierentlyampli- In the proposed system a system of inductances and capacities known as a wave filter is used. This system has the property of passing energy perfectly, that is, presenting zero impedance to certain frequencies andof presenting infiniteimpedance, that is allowing no energy to pass on all other frequencies. The frequencies which this filter will not pass will be passed on to the next tube (or the output) perfectly, having been amplified in voltage by a factor. equal to the voltage amplification factor of the tube. Any wave filter may be used which will pass direct current to the plate of the tube. For instance, in Figure 3 a low pass filter is shown. If it is made with a cut-ofi' frequency at ten cycles for example, every frequency above ten cycles will be equally amplified. Figures 4 and 5 are optional designs of band elimination filters, the condensers and inductors of which may be so chosen'that they will not pass certain frequencies, for example of from one thousand to two thousand in which case all frequencies between one thousand and two thousand cycles will be equally amplified, while the frequencies outside of this band will not I be amplified at all.

It is to be understood that the presentinvention exists, not in the type" of filter used,

prevent a filter being made having a charac- I teristic impedanceequivalent to the grid impedanceof-a tube, allow the impedance of a filter to be easily matched to the plate impedance ofa' tube.

ance of the device intowhich it works.

; In Figure 1 there is shown an illustration Thisis important, as the operation of a'filter depends on its character, istic impedance being equivalent .to imped- 2 Leeefiis of the application of this principle to ordinary amplifying tubes. A-A designates a battery for supplying the heating current for the filaments of the vacuum tubes VT- l and VT-2. B-l3 designates a battery for supplying the plate voltage. Batteries C 1 and C 2 supply through the resistance R 1 and R 2, respectively, the necessary blocking potential so that the tubes will always operate on the straight portion of their characteristic curves.

tors where the current to'be amplified is applied. The condenser C 8 between the conductor 6 and the grid 8 of vacuum tube VT1 is a blocking condenser to prevent directpotentials in the input from disturbing the equilbrium potential of the grid 8.

The battery B supplies current through the.

filter Fl to the plate 9. Variations in voltage on the terminal 6 and 7 are passed through C 3 to the grid 8. This causes sim1-.

lar Variations in the plate current of the tube VT1. If these variations are of a frequency such that they are passed by the filter F1, there will be no appreciable change of the potential at the plate 9. If, however, the frequency of these pulses are such that they are rejected by the filter F-1, the potential at the plate 9 will vary in exact unison with that at the grid 8, but with an amplitude many times that at the grid 8. These amplified variations of potential will pass through the condenser G 4 and vary r the potential on the grid 10 of the .vacuum tube VT-2 and a similar action occurs in the vacuum tube VT2 as hereinbefore explained in the vacuum tube VT-l, so that frequencies which are rejected by the filters F1 and F-2, if they are impressed on the terminals 6 and 7, will appear at the plate 11 of the vacuum tube VT-2, greatly increased in amplitude, and will pass through the condenser C 5 where they may be fed from the terminals 12 and 13 to any device which they are to operate.

Figure 2 involves the same principles as illustrated in Figure 1 with the exception of the introduction of the grids G1 and Gr2, in the vacuum tubes VT1 and VT2, respectively, connected to the battery intermediate terminal marked B1, which increase the amplification of the tubes and decrease the possibility of oscillation feed back in the tubes.

Having thus illustrated and described a certain form of construction and arrangement of parts pertaining to the invention, it is to be understood that any type of wave filter may be used and that the various parts may be arranged in any suitable manner, the present illustration not indicating the only arrangement that may be made ;7 and it is desired to include in-this application for Letters Patent of the United States of America, any and all patentable novelty that exists in the invention disclosed and all that comes within the principle of the invention as set forth in the claims hereinafter mentioned. I

What is claimed is:

1-. A vacuum tube amplifier comprising at least two tubes, said two tubes being coupled from the plate of one to the grid of another by means of a condenser, said plate being 7 connected to its respective filament through The numerals 6 and 'T designate conduc- V a band-elimination filter and a source of voltage,"said grid being connected to its filament in the usual manner, and having a midseriestermination at said plate.

2. A vacuum tube amplifier comprising at least two tubes, said two tubes being coupled from the plate of one to the grid of another by means of a condenser, said plate being connected to its respective filament through a band-elimination filter and a source of voltage, said grid being connected to its filaments in the usual manner, and said bandelimination filter comprising at least one and onehalf sections, the said band-elimination filter being connected to the plate end at a mid-series termination and to the Voltage source at a mid-shunt termination.

3. A vacuum tube amplifier comprising a plate circuit current coupling device consisting of a low-pass filter having a mid series termination adjacent said plate and a parallel utilization circuit passing only alternating current components.

In testimony whereof, I hereunto affix my signature.

WINFIELD W. SALISBURY. 

